Bacterial Structure and Function

Lecture 05 [Notes]
Many environmental factors may affect the shape and other characteristics of bacteria (e.g. pigmentation, growth rate, size)

a. temperature of incubation
b. age of the culture (not an individual organism)
c. composition and concentration of the substrate
d. method of fixing and staining
e. pH and oxygen concentration
f. toxic waste buildup

Basic shape is determined by heredity
1. rod ­ bacillus
2. round ­ coccus
3. spiral ­ spirillum

Shapes are part of a continuum rather than being absolute; some bacteria are naturally pleomorphic (multiple shapes)
Bacteria usually exhibit characteristic morphology in young cultures (24 hours) and on favorable growth media. Young cells are often larger than older cells of the same species

CAPSULES - organized accumulations of gelatinous materials exterior to the cell wall; in contrast to SLIME LAYERS (also known as the GLYCOCALYX) which are unorganized accumulations of similar materials
production is genetically controlled and subject to environmental modification ­ wide range in density, thickness, and adhesiveness among different strains; probably produced by the cell membrane
capsules have varying chemical composition dependent on species: may consist of glucose polymers, complex polysaccharides, amino sugars, sugar acids, polypeptides ­ alone or in combination

1. since capsular material is about 98 % water, capsules probably serve as defense buffers against too rapid influx of water into the cell and also against dehydration i.e. osmotic buffers
2. sometimes associated with pathogenicity
3. confer immunological specificity
4. protects bacteria from ingestion by phagocytic cells (greased pig analogy), and bactericidal factors in body fluids of host

SOME EXAMPLES of encapsulated pathogens:
Bacillus anthracis - anthrax
Clostridium perfringens - gas gangrene, food poisoning
Streptococcus pneumoniae - pneumonia
Klebsiella pneumoniae - pneumonia

CELL WALL - essential to bacterial cell; rigid structure does not allow for expansion
chemically unlike any structure present in animal tissues thus an obvious target for drugs that can attack and kill bacteria without harm to the host
provides strong, rigid structural component that can withstand high osmotic pressure caused by high chemical concentrations of inorganic ions in cell (may be as high as 25 atmospheres of pressure);
Gram ­ cells can usually withstand greater internal atmospheric pressure than Gram + cells

all bacterial cells have common structural component ­ commonly called PEPTIDOGLYCAN, murein, cell wall mucopolysaccharide which consists of amino acids tied into a structure which is not a protein:
Peptidoglycan is a large, sheet­like molecule (like a piece of burlap)which covers the entire cell

Cell Wall Differences: Gram + Thick, 20­80 nanometers, 60­100 % peptidoglycan; all are linear polymers of NAG & NAM but vary in length and composition of peptides used to bridge NAG & NAM
some contain TEICHOIC ACIDS linked to NAM;
highly antigenic, aid in serological identification;
ALL contain GLYCEROL (3 carbon sugar alcohol) which combines with lipid membrane structure and is involved in regulation of normal cell division;
protein is NOT found as a constituent of Gram + cell walls except for Group A streptococci which contain M protein

Gram -­ Thinner, more complex; less peptidoglycan (10­20 %);
exterior to the peptidoglycan is a membrane composed of protein and lipopolysaccharide (fatty acids linked to polysaccharide); together this membrane and the peptidoglycan make up the cell wall; this complex is important because it may cause toxicity in animals, particularly inflammation ­ known as ENDOTOXIN lipopolysaccharide retains its toxicity when dissociated from the protein and may serve in the regulation of ions passing into the cell.

fragile, located interior to cell wall; accounts for 8­10 % of dry weight of the cell
chemically ­ composed of phospholipids (glycerol, fatty acids, and phosphate) with proteins embedded in it;
structurally similar to eukaryotic cell membrane and carries on similar passive and active transport activities
about 40% lipid, 60% protein, less than 10% carbohydrates
invagination of the membrane provides the cell's only means of expanding its surface area
1. in aerobic organisms, it transports electrons and protons released during oxidation of bacterial foodstuffs to oxygen with subsequent formation of water; it also converts energy from such oxidations into chemical energy
2. contains some of the enzymes necessary for synthesis and transport of peptidoglycan, teichoic acid, and outer membrane components
3. secretes extracellular hydrolytic enzymes
4. ensures segregation of DNA to daughter cells during cell division
5. controls transport of most compounds entering & leaving cell (diffusion, osmosis, facilitated & active transport ­ utilizes permease enzymes)

fine, filamentous surface appendages of varying diameters and lengths extending outward from the surface of bacteria ­
can be seen only with electron microscope;
occur most frequently in Gram ­ rods;
originate inside of cell just below the cell membrane;
production is genetically controlled
consist of a unique protein known as PILIN, arranged helically to form a single rigid filament with a hollow core
1. allows for adherence to most surfaces, cellular or otherwise; may cause hemagglutination of human and animal red blood cells
2. attachment of two bacteria prior to transfer of DNA from one to the other ­ requires special longer F pilus
3. allows organisms to form surface film (pellicle) which could enhance microbial growth in non­moving culture situations where oxygen supply is limited
4. used as receptor sites for bacterial viruses (bacteriophages)

MESOSOMES - (probably artifacts of staining procedure) the following is information often found in texts
intracellular membranous structures, first seen in 1957; they appear as pocketlike structures that contain tubules, vesicles or lamellae; chemically identical to the cell membrane
a folded membrane arrangement is found most often in Gram +

1. increase cell's membrane surface, which in turn increases enzymatic needs of cell and formation of new mesosomes
2. mesosome formation may precede and coordinate cross wall formation prior to cell division and spore formation (cell wall and endospore formation)
3. mesosomes may adhere to nuclear material (DNA) and act as a primitive mitotic apparatus to insure that each daughter cell receives appropriate share of DNA; does not appear to be essential for the process to occur
4. increases surface area for respiratory activity
5. may take the place of the ENDOPLASMIC RETICULUM and GOLGI

provides for true motility as opposed to Brownian movement;
threadlike appendage; usually several times the length of the cell; small size ­ cannot be seen in routine stained smears;
special staining methods coat the flagella or cause it to swell

originate just below the cell membrane; attached by a hook and a series of plates or rings which anchor the flagellum to the cell wall and membrane ­ 2­4 rings, only 2 in Gram + cells because of thicker cell wall (only S and M rings)
flagella consist of three parallel protein fibers intertwined into a triple helical structure; protein is called FLAGELLIN (contains an amino acid found nowhere else ­ Epsilon­N­methyl­ lysine); Flagellin is actually a family of proteins rather than a single one
the amino acid composition of Flagellin varies for each species and thus confers immunological specificity for each species
flagella can be broken off and will regenerate; may be dissociated and will repolymerize ­ capable of auto (self) assembly; energy source for flagella is presently unidentified
1. allows organisms to migrate toward favorable growth environments and away from those that might be harmful
2. may increase the concentration of nutrients or decrease the concentration of poisonous materials near bacterial surfaces by causing change of flow rate of environmental fluids
3. can disperse flagellated organisms to uninhabited areas where colony formation can be achieved
4. confer immunological specificity on species
5. may enable flagellated pathogens to more easily penetrate host defenses, such as mucous secretions
the most satisfactory motility is observed in young cultures (18­24 hours or less); bacteria tend to become non­motile in older cultures because of crowding with inert living and dead bacteria; production of acid and other toxic products may cause loss of motility in older cultures

Flagella located at one or both ends of the cell only are called POLAR; if the entire cell surface is covered with flagella it is said to be a PERITRICHOUS condition

prior to the early 1990's it was assumed that bacteria did NOT possess a cytoskeletal network of internal support; in the 1990's evidence emerged that bacteria possessed their own version (homolog) of actin protein; tubulin and intermediate filament homologs have since been found (Naturwissenshaften 1998 Jun;85(6):278-82 and Cell Biology Int. 2003;27(7):503-5 - additional sources may be found through a Google search); current evidence shows that there IS a dynamic cytoskeleton present within the bacterial cytosol

not membrane limited; lacks associated histones; exhibits no mitotic or meiotic phenomena or apparatus; no nucleolus
DNA is contained within a single, long, double­stranded, circular chromosome (closed loop); may have as many as 6 million nucleotide bases (Escherichia coli) - able to produce over 20,000 enzymes; coded information is overlapped within the chromosome (Scientific American, January 1966).

Additional genetic information is found in EXTRACHROMOSOMAL circular DNA molecules known as PLASMIDS; they are capable of independent replication and carry information for a variety of functions, e.g. drug resistance, production of bacteriocins
metabolic PLASMIDS of Pseudomonas produce metabolic enzymes
some PLASMIDS are capable of shifting their mode of replication; they can move back and forth between the bacterial chromosome and their normal site near the cell membrane; when they enter the chromosome they become part of it and are replicated when the bacterium carries on binary fission; as PLASMIDS they can reproduce on demand within the non-dividing cell these PLASMIDS are given different names (to confuse students); they are sometimes called BIPHASIC REPLICONS of EXTRACHROMOSOMAL DNA - the most common term applied to them is EPISOMES; occasionally both PLASMIDS and EPISOMES are referred to as TRANSPOSONS

scattered throughout the cytoplasm; consist of about 40% protein and 60% ribosomal RNA; smaller than those in eukaryotic cells but similar in size to those in the mitochondria and chloroplasts

there is no conspicuous ENDOPLASMIC RETICULUM within the bacterial cell

ribosomes sometimes are found in groups or aggregates known as POLYRIBOSOMES or POLYSOMES

dormant structure formed inside of individual bacterium; resistant to most adverse environmental factors such as: temperature (heat and cold), dessication (dehydration). chemicals, radiation, common dyes
Chemical Composition
little free or unbound water; peptidoglycan, DIPICOLINIC ACID and calcium ions - the last two are usually bound together as calcium dipicolinate (reduced form of the acid with the calcium replacing the hydrogen) DPA = 2,6 pyridine dicarboxylic acid DPA is most responsible for heat resistance core contains characteristic quantity of DNA, small amounts of enzymes and RNA; no messenger RNA; high concentrations of cystine (amino acid) in outer coat - permits cross linkage and has been shown to be effective in resistance to radiation

Sporulation proceeds when conditions are no longer favorable for growth.
The following chemical substances appear to be required for sporulation:
1. glucose - energy source
2. particular amino acids - needed for structural molecules
3. growth factors - vitamins and minerals, including folic acid, phosphate, calcium, manganese, bicarbonate (buffer to maintain pH balance)
Endospore is NOT a reproductive cell; more like fall-out shelter; serves as survival mechanism
Medically important spore formers: the genus Bacillus and the genus Clostridium
other spore formers include:
Sporosarcina - only coccal spore former known; marine organisms
Sporolactobacillus - microaerophilic, found in chicken feed
Desulfomaculum - anaerobe, found in soil, water, some intestines of insects and cows

bacteria accumulate reserve materials - both water soluble and water insoluble; they are non-living and most frequently include lipids, polysaccharides, and certain inorganic materials; these inclusions are divided into two major categories
I. Non-membrane enclosed inclusions
A. Metachromatic granules - sometimes called Babes-Ernst granules or VOLUTIN; they possess an affinity for basic dyes and thus cause color changes within the cell, e.g. these areas turn red when methylene blue is added; they usually contain various associations of phosphates (inor- ganic), nucleic acid, lipids, and proteins; they probably serve as temporary storage of reserve materials
B. Polysaccharide granules - usually insoluble polysaccharides; most commonly glucose polymers

II. Membrane-enclosed Inclusions - surrounded by a single layer of protein
A. Carboxysomes - restricted to photosynthetic cells; contain enzymes responsible for carbon dioxide fixation
B. Lipid Inclusions - appear most regularly in Gram + species, more prominent as the cell ages; usually a polymerized form of the fatty acid B-hydroxybutyric acid (BHA); found in butter, used a food preservative; may become poly b-hydroxybutyric acid and hydrolyzed back to soluble BHA
C. Sulfur Globules - pure elemental sulfur may accumulate as unused food; derived from intracellular dehydrogenation (oxidation) of hydrogen sulfide or other inorganic chemi- cally reduced forms of sulfur
D. Gas Vacuoles - usually in aquatic procaryotes; provides for provision and regulation of cell buoyancy, light- shielding, and surface to volume regulation

Advantages of Storing Food as Polymers
1. many molecules may be condensed into a small area
2. can be made available for cell use by simple hydrolysis into soluble units
3. polymers do not affect the intracellular osmotic pressure or other properties which are normally affected by the number of particles in suspension or solution